Please tell me the actionscript code in flash 5.0 to retrieve computer serial number or HDD serial number or any information which would be unique for a computer so that the flash exe file will run only on one computer.

You could just write a script that nabs some static info off the computer and generates your own HWID based on the variables pulled off the computer, such as mac address and/or a planted flash cookie with a unique ID. Or you could just write a script that pulls the windows HWID and stores it somewhere hashed, compare it against itself at startup, then if they put the software on any other computer it fails. On the other hand, if you are running any of this through a browser you will have an awful time as the browsers now-a-days (especially IE) tend to try and blocking that kind of stuff.